Brazil's Araripe Manakin (Antilophia bokermanni). There are less than 800 of these critically endangered birds left in the wild. They are threatened by habitat destruction due to expansion of agriculture.

One of the threatened species one the list is the pygmy three-toed sloth, which exists only on Escudo Island, off the coast of Panama. At half the size of their mainland cousins, and weighing roughly the same as a newborn baby, pygmy sloths are the smallest and slowest sloths in the world and remain critically endangered. [Image Gallery: 100 Most Threatened Species]

Conservationists fear many of the species on the list will be allowed to die out because the species on the list don't provide humans with obvious benefits, a view that conservation groups are trying to counter.

"The donor community and conservation movement are leaning increasingly towards a 'what can nature do for us' approach, where species and wild habitats are valued and prioritized according to the services they provide for people," said Jonathan Baillie, a conservation scientist with the Zoological Society of London, in a statement. "This has made it increasingly difficult for conservationists to protect the most threatened species on the planet."

But species can be saved if actions are taken, the scientists involved with the list say. Examples include Przewalski's horse and humpback whales, which have been kept around due to aggressive action, according to the release.

"All species have a value to nature and thus in turn to humans," said IUCN scientist Simon Stuart, in the release. "Although the value of some species may not appear obvious at first, all species in fact contribute in their way to the healthy functioning of the planet."